Drawesome.uy - Offers both Drawing & Guessing games as well as collaborative drawing lobbies.
Drawasaurus - Drawasaurus is an online drawing & guessing game you can play with friends or strangers.
DrawPile - Drawpile is a drawing program that lets you share the canvas with other users in real time.
Sketcharama - Draw a given word and let the other players guess your sketch.
Doodle or Die - It's like the game of Telephone. Except with doodles.
skribbl - Word guessing based on drawing.